596 SCIENCE PROGRESS 



year drew special attention to a paper written for students by 

 an American geologist, Professor Chamberlin of Chicago, en- 

 titled "The Method of Multiple Working Hypotheses," in 

 which the danger of interpreting observations in only one way 

 is commented on. 1 A complaint that may be justly made 

 against the workers with radium, I believe, is that they have 

 hitherto allowed themselves to be possessed too entirely by 

 one idea. 



Too little notice, it appears to me, has been taken of the special 

 properties of helium and the other argonides in considering 

 the possible nature of radium. Radium cannot contain helium 

 as we know it, any more than ammonia contains nitrogen as we 

 know it. The nitrogen in ammonia is the active atomic form 

 of nitrogen — unknown in the free state — which combines with 

 itself to form the practically inert molecules we handle when 

 dealing with atmospheric nitrogen gas. If we apply this 

 argument to radium, we can only regard it as a compound 

 containing an active form of helium which in combination with itself 



1 With regard to the use of the term element as applied to radium and similar 

 substances, the tendency in modern times has been towards precision in language. 

 It is the practice to draw a definite distinction between the terms atom and 

 molecule and to confine the latter to the physical or kinetic units or moving masses 

 distinguishable in a substance. Graham's life work was the study of the motions 

 of molecules. The tendency referred to is the outcome of the general recognition 

 of Avogadro's theorem as the one means at our disposal of determining relative 

 molecular weights. Previously the terms atom and molecule were scarcely 

 distinguishable. Whenever Clerk- Maxwell, for example, spoke of atoms, he usually 

 meant molecules — the kinetic units in a gas. Graham, in his early work on salts, 

 following the practice of his time, speaks of atoms of water. Whilst the modern 

 chemist regards the molecule as the physical or kinetic unit, he regards the atom 

 as the chemical unit — the unit active within the molecule of a compound — and he 

 speaks of groups of atoms which act as combining units as compound radicles, 

 regarding atoms as simple radicles. But the atom has long been looked upon as 

 the undivided quantity, not as the indivisible quantity. Moreover, whilst the 

 chemist has long been willing to admit that the properties of the elements are 

 those of a series of genetically, more or less closely related compounds, he has 

 always thought of them as compounds of an order very different from that to 

 which those commonly known belong. None the less, the term element has a 

 very restricted meaning in his mind and can scarcely be applied to a substance 

 proved to be composite : an essential connotation of the term is undoubtedly that 

 of a primary substance in the strictest sense — a substance that has been resolved 

 has parted with the right to rank as an element proper. To argue otherwise 

 would be illogical : it would be as justifiable to apply the term element to benzene 

 or any other unsaturated compound capable of combining as a whole, as it is to 

 speak of radium as an element now that it is shown to be compound. We need, 

 in fact, a special name for substances such as radium. 



